C# error CS0246: The type or namespace name could not be found

error CS0246: The type or namespace name 'X' could not be found (are you missing a using directive or an assembly reference?) means the C# compiler cannot find a definition for X. Either the assembly that defines it is not referenced (missing NuGet package or project reference), or its namespace is not imported with using. Add the package or reference, then the using directive.

Reproduce it

export DOTNET_NOLOGO=1 DOTNET_CLI_TELEMETRY_OPTOUT=1
dotnet new console -o app >/dev/null 2>&1
cd app
cat > Program.cs <<'EOF'
using Newtonsoft.Json;
Console.WriteLine(JsonConvert.SerializeObject(new { ok = true }));
EOF
dotnet build -nologo 2>&1 | grep -o 'error CS[0-9]*: [^[]*' | sort -u
exit ${PIPESTATUS[0]}

error CS0246: The type or namespace name 'Newtonsoft' could not be found (are you missing a using directive or an assembly reference?)

The using line fails because no referenced assembly contains a Newtonsoft namespace. The message names the first part of the name that cannot be resolved, here the namespace rather than JsonConvert.

Fix: reference the package

dotnet add package Newtonsoft.Json
dotnet build

When the type exists but only the namespace import is missing, the error is the same and the fix is the using line. The project file then contains:

<ItemGroup>
  <PackageReference Include="Newtonsoft.Json" Version="13.0.4" />
</ItemGroup>

Checklist